s4:samba-tool: Fix samba-tool fsmo --role=schema

s4:samba-tool: Fix samba-tool fsmo --role=schema

Fix traceback:
samba-tool fsmo --role=schema --force
ERROR(<type 'exceptions.TypeError'>): uncaught exception - argument 2 must be string, not ldb.Dn
  File "/usr/lib/python2.6/dist-packages/samba/netcmd/__init__.py", line 168, in _run
    return self.run(*args, **kwargs)
  File "/usr/lib/python2.6/dist-packages/samba/netcmd/fsmo.py", line 160, in run
    self.seize_role(role, samdb, force)
  File "/usr/lib/python2.6/dist-packages/samba/netcmd/fsmo.py", line 119, in seize_role
    m.dn = ldb.Dn(samdb, self.schema_dn)

Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Fri Nov 16 00:40:24 CET 2012 on sn-devel-104
(cherry picked from commit ace0909b88739338e948b9c1e98307f324bb7503)

Fix bug #9392 - Traceback in samba-tool fsmo --role=schema.

Autobuild-User(v4-0-test): Karolin Seeger <kseeger@samba.org>
Autobuild-Date(v4-0-test): Fri Nov 16 10:54:29 CET 2012 on sn-devel-104